It is known that this ground effect release speed (generally referred to by French speaking technicians as VMU speed, for Vitesse Minimum Unstick, and by English speaking technicians as Minimum Unstick Speed) is used in calculating the take-off performance of an aircraft, imposing the runway length being required upon take-off thereof or the take-off mass of such aircraft for a given runway length. Thus, the lower the ground effect release speed is, the better the take-off performance of the aircraft is. There is thus generally some interest to try to minimize such ground effect release speed.
In order to determine the ground effect release speed of an aircraft, it is known to perform a non regular take-off as follows: while said aircraft is rolling down on a take-off runway, accelerating with the main landing gear thereof compressed by the mass of said aircraft and by the ground effect and with the ailerons having a position which is partly deflected downwards in order to maximise the fineness ratio of said aircraft:
by tilting the piloting stick in the nose-up direction, said depth aerodynamic surfaces are deflected upwards for giving said aircraft the attitude for which the tail thereof is brought to strike said take-off runway;
in this tail strike attitude position, the acceleration of said aircraft is carried on until full ballasting-up of said main landing gear; and
the speed of said aircraft upon said ballasting-up is noticed, such speed being the desired ground effect release speed.
The depth aerodynamic surfaces used in this method are the horizontal rudders and/or an incidence adjustable stabiliser, generally referred to as either the following abbreviations: PHP. (for Plan Horizontal Réglable in French) or THS (for Trimmable Horizontal Stabiliser). In addition, in order to avoid any damage against the take-off runway, the tail of said aircraft is provided with a protecting pad, at the low point thereof.